MAPPING THE RECENT STAR FORMATION HISTORY OF THE DISK OF M51

Abstract

Using data acquired as part of a unique Hubble Heritage imaging program of broadband colors of the interacting spiral system M51/NGC 5195, we have conducted a photometric study of the stellar associations across the entire disk of the galaxy in order to assess trends in size, luminosity, and local environment associated with recent star formation activity in the system. Starting with a sample of over 900 potential associations, we have produced color-magnitude and color-color diagrams for the 120 associations that were deemed to be single-aged. It has been found that main sequence turnoffs are not evident for the vast majority of the stellar associations in our set, potentially due to the overlap of isochronal tracks at the high mass end of the main sequence, and the limited depth of our images at the distance of M51. In order to obtain ages for more of our sample, we produced model spectral energy distributions (SEDs) to fit to the data from the GALEXEV simple stellar population (SSP) models of Bruzual and Charlot (2003). These SEDs can be used to determine age, size, mass, metallicity, and dust content of each association via a simple chi-squared minimization to each association's B, V, and I-band fluxes. The derived association properties are mapped as a function of location, and recent trends in star formation history of the galaxy are explored in light of these results. This work is the first phase in a program that will compare these stellar systems with their environments using ultraviolet data from GALEX and infrared data from Spitzer, and ultimately we plan to apply the same stellar population mapping methodology to other nearby face-on spiral galaxies.